Antenna device

ABSTRACT

An antenna device includes: an antenna configured to radiate a radio wave; a substrate at which the antenna is provided; and a cover which covers the substrate from a radiation surface side of the antenna, the cover is a dielectric material, a maximum distance between the cover and the substrate in a normal direction to the substrate is smaller than ½ of a free space wavelength of the radio wave, and a thickness of the cover is smaller than ½ of an effective wavelength of the radio wave in the cover.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Application No. 2019-098551 filed on May 27, 2019.

TECHNICAL FIELD

The present invention relates to an antenna device.

BACKGROUND ART

An antenna device is widely known which uses a structure including acover that covers a substrate from a radiation surface side of theantenna so as to protect an antenna provided at the substrate (forexample, see International Publication No. WO 2005/055366). In theantenna device mounted at a radar device, the cover is generallyreferred to as a radome.

SUMMARY OF INVENTION

In the above-described antenna device including the cover, the cover mayadversely affect a radiation pattern of the antenna device, and theradiation pattern may be distorted. For example, in the antenna devicemounted at the radar device, as the distortion of the radiation patternbecomes larger, angular resolution of the radar device and a minimumdetection distance of the radar device become lower.

An aspect of the present invention provides an antenna device capable ofpreventing a radiation pattern from being distorted.

An antenna device according to an aspect of the present inventioncomprises: an antenna configured to radiate a radio wave; a substrate atwhich the antenna is provided; and a cover configured to cover thesubstrate from a radiation surface side of the antenna, wherein thecover is a dielectric material, wherein a maximum distance between thecover and the substrate in a normal direction of the substrate issmaller than ½ of a free space wavelength of the radio wave, and whereina thickness of the cover is smaller than ½ of an effective wavelength ofthe radio wave in the cover (first configuration).

It may be that the antenna device according to the first configurationfurther comprises an interposition member interposed between the coverand the substrate, wherein a dielectric constant of the interpositionmember is different from both a dielectric constant of the substrate anda dielectric constant of the cover, and wherein a maximum distancebetween the cover and the substrate in the normal direction of thesubstrate is smaller than ½ of an effective wavelength of the radio wavein the interposition member (second configuration).

It may be that the antenna device according to the first configurationfurther comprises a ground pattern provided at the substrate, wherein amaximum distance between the cover and the ground pattern in the normaldirection of the substrate is smaller than ½ of a free space wavelengthof the radio wave (third configuration).

It may be that the antenna device according to the third configurationfurther comprises an interposition member interposed between the coverand the substrate, wherein a dielectric constant of the interpositionmember is different from both a dielectric constant of the substrate anda dielectric constant of the cover, and wherein a maximum distancebetween the cover and the ground pattern in the normal direction of thesubstrate is smaller than ½ of an effective wavelength of the radio wavein the interposition member (fourth configuration).

It may be that, in the antenna device according to the firstconfiguration, the substrate includes a covered surface covered with thecover and an opposite surface opposite to the covered surface, and amaximum distance between the cover and the opposite surface in thenormal direction of the substrate is smaller than ½ of a free spacewavelength of the radio wave (fifth configuration).

It may be that the antenna device according to the fifth configurationfurther comprises an interposition member interposed between the coverand the substrate, wherein a dielectric constant of the interpositionmember is different from both a dielectric constant of the substrate anda dielectric constant of the cover, and wherein a maximum distancebetween the cover and the opposite surface in the normal direction ofthe substrate is smaller than ½ of an effective wavelength of the radiowave in the interposition member (sixth configuration).

It may be that, in the antenna device according to any one of the firstto sixth configurations, the antenna device is an in-vehicle antennadevice, and the antenna is a vertically polarized antenna (seventhconfiguration).

According to the antenna device of the present invention, the radiationpattern can be prevented from being distorted.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of an antenna device according toan embodiment.

FIG. 2 shows an example of a radiation pattern of the antenna deviceshown in FIG. 1.

FIG. 3 shows a radiation pattern according to a first reference example.

FIG. 4 shows a radiation pattern according to a second referenceexample.

FIG. 5 shows a radiation pattern according to a third reference example.

FIG. 6 shows a radiation pattern according to a fourth referenceexample.

FIG. 7 illustrates a first modification of the antenna device.

FIG. 8 illustrates a second modification of the antenna device.

FIG. 9 illustrates a third modification of the antenna device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an exemplary embodiment of the present invention will bedescribed in detail with reference to the drawings. In the presentspecification, “parallel” may not be strictly parallel, and may beregarded as parallel in consideration of a design error, a variation,and the like. In the present specification, “vertical” may not bestrictly vertical, and may be regarded as vertical in consideration of adesign error, a variation, and the like.

FIG. 1 is a schematic sectional view of an antenna device according tothe embodiment. In FIG. 1, orthogonal coordinates of an X axis, a Yaxis, and a Z axis are also shown so as to facilitate understanding of adescription. The orthogonal coordinates may also be shown in otherdrawings used in the following description.

An antenna device 1 illustrated in FIG. 1 includes a transmissionantenna 2, a substrate 3, and a cover 4.

The transmission antenna 2 is configured to radiate a radio wave W. Theradio wave W radiated from the transmission antenna 2 is radiated to anoutside of the antenna device 1 through the cover 4.

An electric field component of the radio wave W propagates with anamplitude in a direction parallel to the Z axis. Unlike the presentembodiment, the transmission antenna 2 may radiate a radio wave whoseelectric field component propagates with an amplitude in a direction notparallel to the Z axis. However, in this case, an effect of preventing aradiation pattern from being distorted is weakened as compared with thatof the present embodiment.

One transmission antenna 2 is illustrated in FIG. 1. However, the numberof transmission antennas 2 provided in the antenna device 1 may be oneor plural. The transmission antenna 2 is provided at the substrate 3.

As the transmission antenna 2, for example, a radiation element of amicrostrip antenna may be used. When the transmission antenna 2 is theradiation element of the microstrip antenna, a ground pattern facing theradiation element is provided on the substrate 3. For example, a slotantenna may be formed in the substrate 3, and the slot antenna may beused as the transmission antenna 2.

A distance to a target, an azimuth (an angle) of the target, a relativespeed of the target, and the like may be detected by a reception antennareceiving a reflected wave. The reflected wave is obtained by the targetreflecting the radio wave W radiated from the antenna device 1. Thereception antenna may be provided in the antenna device 1 or may beprovided outside the antenna device 1. The transmission antenna 2 mayalso serve as the reception antenna. Thus, the transmission antenna 2may be a transmission and reception antenna configured to transmit theradio wave W and to receive the reflected wave.

The substrate 3 is formed of a dielectric material which is resin orTeflon (registered trademark), for example. The substrate 3 includes acovered surface F1 covered with the cover 4 and an opposite surface F2opposite to the covered surface F1. The covered surface F1 and theopposite surface F2 are outer surfaces of the substrate 3. A normaldirection of the substrate 3, more specifically, normal directions ofthe covered surface F1 and the opposite surface F2 are parallel to the Yaxis.

The cover 4 is configured to cover the substrate 3 from a radiationsurface side of the transmission antenna 2. The cover 4 is a protectionmember configured to protect the transmission antenna 2 from naturalenvironment which is wind, rain, sand, and the like. The cover 4 isformed of a dielectric material which is glass fiber or Teflon, forexample. Air is present in a space surrounded by the cover 4 and thecovered surface F1 of the substrate 3 except for a region occupied by anelement which is the antenna 2 or the like provided on the coveredsurface F1 of the substrate 3.

A maximum distance d1 (hereinafter, abbreviated as a “distance d1”)between the cover 4 and the substrate 3 in the normal direction of thesubstrate 3 is smaller than ½ of a free space wavelength λ of the radiowave W. Accordingly, the distance d1 is smaller than a cutoff wavelengthof the radio wave W propagating along the X axis between the cover 4 andthe substrate. As a result, the radio wave W may be prevented frompropagating along the X axis between the cover 4 and the substrate.

If a part of the radio wave W propagating along the X axis between thecover 4 and the substrate leaks to the outside of the antenna device 1in a direction including a positive direction component of the Y axis, aradiation pattern of the antenna device 1 is distorted. The antennadevice 1 may prevent the radio wave W from propagating along the X axisbetween the cover 4 and the substrate as described above, so that theantenna device 1 may prevent a radiation pattern in a direction parallelto the X axis from being distorted.

For example, when the cover 4 has a hemispherical shape unlike thepresent embodiment, a distance between a vertex of a hemisphere and thesubstrate 3 is the distance d1.

A thickness t1 of the cover is smaller than ½ of an effective wavelengthλg of the radio wave W in the cover 4. The effective wavelength is awavelength of a radio wave when the radio wave propagates through amedium. Accordingly, the thickness t1 is smaller than a cutoffwavelength of the radio wave W propagating along the X axis inside thecover 4. As a result, the radio wave W may be prevented from propagatingalong the X axis inside the cover 4.

If a part of the radio wave W propagating along the X axis inside thecover 4 leaks to the outside of the antenna device 1 in the directionincluding the positive direction component of the Y axis, the radiationpattern of the antenna device 1 is distorted. The antenna device 1 mayprevent the radio wave W from propagating along the X axis inside thecover 4 as described above, so that the antenna device 1 may prevent theradiation pattern in the direction parallel to the X axis from beingdistorted.

Unlike the present embodiment, when a thickness of the cover 4 is notuniform, a maximum thickness excluding a thickness t2 in a directionperpendicular to the Y axis may be smaller than ½ of the effectivewavelength λg of the radio wave W in the cover 4. If a characteristic ofthe propagation of the radio wave W along the X axis inside the cover 4in a case in which the cover 4 has a laminated structure cannot beregarded as equivalent to that in a case in which the cover 4 has asingle layer structure, each layer of the laminated structure isconsidered as a separate cover, and a thickness of each cover may besmaller than ½ of the effective wavelength λg of the radio wave W in thecover 4.

FIG. 2 shows an example of the radiation pattern of the antenna device 1in the direction parallel to the X axis. The radiation pattern shown inFIG. 2 is almost not distorted, and has a characteristic close to thatof a radiation pattern (a radiation pattern according to the firstreference example shown in FIG. 3) of an antenna device (an antennadevice having a structure in which the cover 4 is removed from theantenna device 1) not including a cover.

For example, when the antenna device 1 is an in-vehicle antenna deviceand is mounted at an in-vehicle radar device, the antenna device 1 ismounted in a vehicle which is an automobile or the like such that the Xaxis and the Y axis are horizontal with respect to a ground and the Zaxis is perpendicular to the ground. As described above, thetransmission antenna 2 is configured to radiate the radio wave W whoseelectric field component propagates with an amplitude in a directionparallel to the Z axis, so that the transmission antenna 2 is avertically polarized antenna when the antenna device 1 is mounted in thevehicle which is the automobile or the like such that the Z axis isperpendicular to the ground. With this configuration, for example,angular resolution of the in-vehicle radar device in the horizontaldirection and a minimum detection distance of the in-vehicle radardevice in the horizontal direction may be improved.

FIGS. 4 to 6 shows radiation patterns according to second to fourthreference examples. In the radiation patterns according to the second tothe fourth reference examples shown in FIGS. 4 to 6, distortions cannotbe sufficiently prevented.

The radiation pattern according to the second reference example shown inFIG. 4 is an example of a case in which the distance d1 is larger than ½of the free space wavelength λ of the radio wave W and the thickness t1of the cover 4 is larger than ½ of the effective wavelength λg of theradio wave W in the cover 4. The radiation pattern according to thethird reference example shown in FIG. 5 is an example of a case in whichthe thickness t1 of the cover 4 is not changed from that of theradiation pattern according to the second reference example shown inFIG. 4 and the distance d1 is smaller than ½ of the free spacewavelength λ of the radio wave W. It can be seen from FIG. 5 that thedistortion may be prevented by causing the distance d1 to be smallerthan ½ of the free space wavelength λ of the radio wave W, but thiscountermeasure alone is insufficient.

The radiation pattern according to the fourth reference example shown inFIG. 6 is another example of a case in which the distance d1 is smallerthan ½ of the free space wavelength λ of the radio wave W and thethickness t1 of the cover 4 is larger than ½ of the effective wavelengthλg of the radio wave W in the cover 4. The radiation pattern accordingto the third reference example shown in FIG. 5 and the radiation patternaccording to the fourth reference example shown in FIG. 6 have differentvalues of the thickness t1 of the cover 4.

An example of the radiation pattern shown in FIG. 2 is an example of acase in which the distance d1 is not changed from that of the radiationpattern according to the fourth reference example shown in FIG. 6 andthe thickness of the cover 4 is smaller than ½ of the effectivewavelength λg of the radio wave W in the cover 4. It can be seen fromFIG. 2 that the distortion can be sufficiently prevented by causing thethickness of the cover 4 to be smaller than ½ of the effectivewavelength λg of the radio wave W in the cover 4 in addition to causingthe distance d1 to be smaller than ½ of the free space wavelength λ ofthe radio wave W.

For example, when a dielectric constant of the substrate 3 is relativelylow and a structure (not illustrated) formed of a conductor is incontact with the opposite surface F2, the radio wave W may propagatealong the X axis between the cover 4 and the structure in contact withthe opposite surface F2. Therefore, a maximum distance d2 (hereinafter,abbreviated as a “distance d2”) between the cover 4 and the oppositesurface F2 in the normal direction of the substrate 3 is preferablysmaller than ½ of the free space wavelength λ of the radio wave W.Accordingly, the distance d2 is smaller than a cutoff wavelength of theradio wave W propagating along the X axis between the cover 4 and thestructure in contact with the opposite surface F2. As a result, theradio wave W may be prevented from propagating along the X axis betweenthe cover 4 and the structure in contact with the opposite surface F2.

FIG. 7 illustrates a first modification of the antenna device 1. Anantenna device 1A illustrated in FIG. 7 has a configuration in which aninterposition member 5 is added to the antenna device 1. Theinterposition member 5 is interposed between the cover 4 and thesubstrate 3. The interposition member 5 occupies a space (except for theregion occupied by the element which is the antenna 2 or the likeprovided on the covered surface F1 of the substrate 3) surrounded by thecover 4 and the covered surface F1 of the substrate 3. The interpositionmember 5 may improve a mechanical strength of the antenna device 1A.

The interposition member 5 is formed of, for example, resin. Adielectric constant of the interposition member 5 is different from boththe dielectric constant of the substrate 3 and a dielectric constant ofthe cover 4. In the antenna device 1A, the radio wave W may propagatealong the X axis between the cover 4 and the substrate 3, in otherwords, inside the interposition member 5. Therefore, the distance d1 ispreferably smaller than ½ of an effective wavelength λg′ of the radiowave W in the interposition member 5. Accordingly, the radio wave W maybe prevented from propagating along the X axis inside the interpositionmember 5, so that the radiation pattern in the direction parallel to theX axis may be prevented from being distorted.

In the antenna device 1A, as in the antenna device 1, for example, whenthe dielectric constant of the substrate 3 is relatively low and astructure (not illustrated) formed of a conductor is in contact with theopposite surface F2, the radio wave W may propagate along the X axisbetween the cover 4 and the structure in contact with the oppositesurface F2. Therefore, the distance d2 is preferably smaller than ½ ofthe effective wavelength λg′ of the radio wave W in the interpositionmember 5. Accordingly, the distance d2 is smaller than the cutoffwavelength of the radio wave W propagating along the X axis between thecover 4 and the structure in contact with the opposite surface F2. As aresult, the radio wave W may be prevented from propagating along the Xaxis between the cover 4 and the structure in contact with the oppositesurface F2.

FIG. 8 illustrates a second modification of the antenna device 1. Anantenna device 1B illustrated in FIG. 8 has a configuration in which aground pattern 6 is added to the antenna device 1. The ground pattern 6is provided at the opposite surface F2 of the substrate 3. Unlike thepresent modification, the ground pattern 6 may be provided in thesubstrate 3. The ground pattern 6 is formed of a conductor which iscopper, copper alloy, aluminum, or aluminum alloy. The ground pattern 6is grounded.

For example, when the dielectric constant of the substrate 3 isrelatively low, the radio wave W may propagate along the X axis betweenthe cover 4 and the ground pattern 6. Therefore, a maximum distance d3(hereinafter, abbreviated as a “distance d3”) between the cover 4 andthe ground pattern 6 in the normal direction of the substrate 3 ispreferably smaller than ½ of the free space wavelength λ of the radiowave W. Accordingly, the distance d3 is smaller than a cutoff wavelengthof the radio wave W propagating along the X axis between the cover 4 andthe ground pattern 6. As a result, the radio wave W may be preventedfrom propagating along the X axis between the cover 4 and the groundpattern 6.

FIG. 9 illustrates a third modification of the antenna device 1. Anantenna device 1C illustrated in FIG. 9 has a configuration in which aninterposition member 5 is added to the antenna device 1B. Theinterposition member 5 provided in the antenna device 1C is the same asthe interposition member 5 provided in the antenna device 1A, so that adetailed description thereof will be omitted.

In the antenna device 1C, the radio wave W may propagate along the Xaxis inside the interposition member 5. Therefore, the distance d3 ispreferably smaller than ½ of the effective wavelength λg′ of the radiowave W in the interposition member 5. Accordingly, the radio wave W maybe prevented from propagating along the X axis inside the interpositionmember 5, so that the radiation pattern in the direction parallel to theX axis may be prevented from being distorted.

Configurations of the embodiments in the present specification aremerely examples of the present invention. The configurations of theembodiments and the modifications may be modified as appropriate withoutdeparting from the technical idea of the present invention. A pluralityof the embodiments and the modifications may be implemented incombination within a possible range.

What is claimed is:
 1. An antenna device comprising: an antennaconfigured to radiate a radio wave; a substrate at which the antenna isprovided; and a cover which covers the substrate from a radiationsurface side of the antenna, wherein the cover is a dielectric material,wherein a maximum distance between the cover and the substrate in anormal direction to the substrate is smaller than ½ of a free spacewavelength of the radio wave, and wherein a thickness of the cover issmaller than ½ of an effective wavelength of the radio wave in thecover.
 2. The antenna device according to claim 1, further comprising aninterposition member interposed between the cover and the substrate,wherein a dielectric constant of the interposition member is differentfrom both a dielectric constant of the substrate and a dielectricconstant of the cover, and wherein the maximum distance between thecover and the substrate in the normal direction to the substrate issmaller than ½ of an effective wavelength of the radio wave in theinterposition member.
 3. The antenna device according to claim 1,further comprising a ground pattern provided at the substrate, wherein amaximum distance between the cover and the ground pattern in the normaldirection to the substrate is smaller than ½ of a free space wavelengthof the radio wave.
 4. The antenna device according to claim 3, furthercomprising an interposition member interposed between the cover and thesubstrate, wherein a dielectric constant of the interposition member isdifferent from both a dielectric constant of the substrate and adielectric constant of the cover, and wherein the maximum distancebetween the cover and the ground pattern in the normal direction to thesubstrate is smaller than ½ of an effective wavelength of the radio wavein the interposition member.
 5. The antenna device according to claim 1,wherein the substrate has a covered surface covered with the cover andan opposite surface opposite to the covered surface, and wherein amaximum distance between the cover and the opposite surface in thenormal direction to the substrate is smaller than ½ of a free spacewavelength of the radio wave.
 6. The antenna device according to claim5, further comprising an interposition member interposed between thecover and the substrate, wherein a dielectric constant of theinterposition member is different from both a dielectric constant of thesubstrate and a dielectric constant of the cover, and wherein themaximum distance between the cover and the opposite surface in thenormal direction to the substrate is smaller than ½ of an effectivewavelength of the radio wave in the interposition member.
 7. The antennadevice according to claim 1, wherein the antenna device is an in-vehicleantenna device, and wherein the antenna is a vertically polarizedantenna.
 8. The antenna device according to claim 2, wherein the antennadevice is an in-vehicle antenna device, and wherein the antenna is avertically polarized antenna.
 9. The antenna device according to claim3, wherein the antenna device is an in-vehicle antenna device, andwherein the antenna is a vertically polarized antenna.
 10. The antennadevice according to claim 4, wherein the antenna device is an in-vehicleantenna device, and wherein the antenna is a vertically polarizedantenna.
 11. The antenna device according to claim 5, wherein theantenna device is an in-vehicle antenna device, and wherein the antennais a vertically polarized antenna.
 12. The antenna device according toclaim 6, wherein the antenna device is an in-vehicle antenna device, andwherein the antenna is a vertically polarized antenna.